The formalin test in rat: validation of an automated system

A novel, computer-driven, dynamic-force detector was validated for use in measuring the formalin-induced agitation response. Intraplantar administration of formalin (0.25-5%) provoked a biphasic agitation response as measured with the automated system. The magnitude of both phases of the response increased with the intensity of the noxious stimulus. Morphine (1-5 mg/kg, s.c.) inhibited both phase 1 and 2 of the agitation response evoked by 5% formalin with ID50 values of 2.07 +/- 0.47 and 1.35 +/- 0.02 mg/kg, respectively. The non-peptide, NK1 antagonist, (+/-)-CP-96,345 (30 mg/kg, s.c.), partially blocked phase 2 but did not alter the magnitude of phase 1. These results are comparable with those obtained by us and others using a multiple-pain-behavior scoring system or certain uni-dimensional measures. In addition, they indicate that the automated system yields a valid measure of the formalin-induced agitation response.